The present invention relates to a charge pump circuit for a high side drive circuit and a driver driving voltage circuit, which are applicable to bridge configurations, such as a half bridge and a full bridge.
Bridge configurations, such as a full bridge and a half bridge, are used as drive circuits for driving motors and other devices. Japanese Laid-Open Patent Publication No. 2000-82946 describes a drive circuit having a full bridge (H bridge) configuration that is effective for use in a wide range of power supply voltages. Such a full bridge drive circuit includes a high side metal oxide semiconductor (MOS) transistor and a low side MOS transistor. The drain terminal of the high side MOS transistor is supplied with power supply voltage for driving a load. The source terminal of the low side MOS transistor is grounded. A drive output terminal OP is formed at a connecting node between the high side MOS transistor and the low side MOS transistor. The subject that is to be driven (drive subject) is connected to the drive output terminal OP. Further, a pre-driver circuit is connected to the gate of each MOS transistor.
The voltage supplied to the gate of each MOS transistor may be raised using a charge pump circuit including a plurality of capacitors. Such a charge pump circuit is used to supply voltage to a circuit that requires voltage that is higher than the supply voltage, such as a motor driver. FIG. 6 shows an example of the charge pump circuit. The charge pump circuit includes capacitors C11 and C12, and switches S1, S2, and S3. At time CLK, the switch S1 is activated, the switch S2 is inactivated, and the switch S3 is grounded. This charges the capacitor 11 and generates a capacitor charge voltage Vc. The capacitor charge voltage Vc is controlled by changing the length of the time CLK. When the time CLK is long enough, the capacitor charge voltage Vc is equal to input voltage Vin (Vc=Vin).
At time CLKB, the switch S1 is inactivated, the switch S2 is activated, and the switch S3 is switched to the side of the voltage Vin so that the capacitor charge voltage Vc and the input voltage Vin are in series. As a result, the output voltage is equal to the sum of the capacitor charge voltage Vc and the input voltage Vin (Vout=Vc+Vin). When the capacitor charge voltage Vc is equal to the input voltage Vin (Vc=Vin), the charge pump circuit outputs an output voltage that is two times greater than the input voltage (2vin). Further, when the charge pump circuit includes n stages of capacitors, the circuit outputs an output voltage that is n+1 times greater than the input voltage ((n+1)Vin).
The switches S1, S2, and S3 are usually formed by MOS transistors, which have low on-resistance. However, in a charge pump circuit used to drive a load with a high power supply voltage, the gate to source voltage of a MOS transistor may become extremely high and cause gate oxide breakdown. To prevent this, diodes may be used as the switches S1, S2, and S3 in lieu of MOS transistors.
However, when diodes are used, rising voltage (Vd) causes a voltage drop (Vd). This does not occur when using MOS transistors. Voltage drops occurring in the switches S1 and S2 amount to 2 Vd. The load driving power supply voltage is normally greater than 2 Vd. Thus such a voltage drop would not cause any problems. However, when the load driving power supply voltage decreases, the voltage drop may disable the driving of the high side charge pump circuit. In such a case, the threshold voltage for a transistor in a high side drive circuit would not be obtained. This would destabilize switching.